1. Field of the Invention
The present invention relates to a sheet conveying apparatus which conveys a sheet and an image forming apparatus which is provided with the sheet conveying apparatus.
2. Description of the Related Art
Conventionally, a sheet conveying apparatus which conveys a sheet to an image forming portion is provided in an image forming apparatus such as a copying machine, a printer, and a facsimile machine. As such an image forming apparatus of an electrophotographic system, there is provided an apparatus suitable for a light printing market (Print On Demand (POD)) in which a small number of printing times are performed while keeping a merit in that no plate is necessary compared to an offset printing machine. However, in order to be credited for such a light printing market, high productivity, high durability, high quality, and processibility of various types of sheets are required.
A sheet is conveyed toward an image forming portion by the sheet conveying apparatus of the image forming apparatus. At this time, when skew feeding of the sheet or a deviation from a position (a lateral registration position) in a width direction perpendicular to the sheet conveying direction occurs, an image is formed in a state that the imaging position is deviated.
Therefore, the sheet conveying apparatus is provided with a skew feeding correction portion which corrects the skew feeding of the sheet and adjusts the lateral registration position on the upstream side in the sheet conveying direction of the image forming portion. As an example of such a skew feeding correction portion, U.S. Patent Application Publication No. 2005/242493 A1 proposes a configuration in which the positional deviation at the side end of a conveying sheet is corrected on the basis of a side registration.
As an example of processibility of various types of sheets, thick sheet processing is exemplified. Generally, a printer or a multifunction peripheral processes a sheet as heavy as about a basis weight of 250 g/m2. When a sheet having a basis weight of 300 g/m2 or more is processed, the apparatus can also be applied to markets such as POP (Point Of Purchase advertising) advertisement printing and package printing.
Since stiffness of the sheet is increased with respect to the basis weight (thickness) in an accelerating manner, specifically, when the sheet is conveyed in a bent state, a conveying force required for the bent portion is also significantly increased. Even in U.S. Patent Application Publication No. 2005/242493 A1, since the sheet is conveyed in an upwardly bent state over a range from a registration roller to a secondary transfer portion, the conveying force is necessarily increased.
Specifically, in order to obtain sheet conveying accuracy of the registration roller with respect to the conveying direction, a driving roller may be made of a material such as metal which easily achieves accuracy of an outer diameter and a driven roller may be made of rubber. However, since the metal roller has a low friction coefficient, a large nipping force is necessary for obtaining the conveying force. Further, in order to avoid a conveying error caused by rippling in a direction perpendicular to the conveying direction of the sheet which is not yet transferred, the registration roller may be made longer than the width of the sheet and configured to nip the whole range of the sheet in the width direction.
FIGS. 20 and 21 are diagrams illustrating a pair of registration rollers 5 of a comparative example when viewed from the sheet conveying direction. In the drawings, a bending state when a follower roller 2 is pressurized in order to obtain a nipping force of the pair of registration rollers 5 is illustrated. The follower roller 2 is provided to face a driving roller 1 which is rotatably driven, and bearing members 3 are disposed in both end portions of the respective rollers 1 and 2 such that the respective rollers 1 and 2 are freely rotated.
The bearing member 3 provided in the driving roller 1 is supported to an apparatus frame (not illustrated) by a fulcrum A of FIG. 20, and the bearing member 3 provided in the follower roller 2 is pressurized toward the driving roller 1 in a direction of arrow F of FIG. 20. In this case, the respective rollers 1 and 2 each receive rotational moments in directions of arrows R1 and R2 of FIG. 20 about points P on the outer side from end portions of a sheet 4a in the width direction, and thus are bent in a reverse direction to each other.
Therefore, a large urging force is applied in a direction of arrow F of FIG. 20 in order to secure the conveying force of the sheet 4a made of thick paper. In this state, when a thin and narrow sheet 4b is conveyed as illustrated in FIG. 21, the nipping force for the sheet 4b becomes insufficient, and thus a conveyance fail may occur.
The invention has been made to solve the above problems, and it is desirable to provide a sheet conveying apparatus which can secure a conveying force with respect to various types of sheets without raising an urging force to pressurize a follower rotating member toward a conveying rotating member.